Large quantities of agricultural chemicals have now been used to prevent microbial infections while crops are cultivated. For example, seeds are treated with chemicals before being sowed in order to promote good germination and furthermore, chemicals are spread in order to reduce damages caused by microbial infections while plants have grown up and bear fruits. However, there are various problems in respect of safety, such as influences on human bodies exerted by the chemicals remaining on the crop, and the environmental pollution by diffused chemicals.
It has been known that a plant has various defense systems against infections originally, and that when a plant is infected with a microorganism, an antimicrobial substance which is not present in normal tissue is induced for resisting the infection. This antimicrobial substance is referred to as a phytoalexin. A phytoalexin is induced by a cell component of a microorganism, a fragment of a polysaccharide which construct the cell wall of the plant infected by a microorganism, or the like. This substance which induces a phytoalexin is referred to as an elicitor.
Moreover, it is considered that the elicitor activity not only induces a phytoalexin but also participates in induction of an enzyme inhibitory substance such as a protease inhibitor, and in production of a plant hormone, e.g., ethylene, and the like.
A fragment of .beta.-glucan derived from the cell wall of Phytophthora megasperma (Janice K. Sharp et al, Journal of Biological Chemistry, Vol. 259, No. 18, 11312-11320, 1984), and oligogalacuturonic acid, which is a fragment of a pectin, (Peter Albersheim et al, Scientific American, Vol. 253, 44-50, 1985; Akira Misaki et al, Agricultural and Biological Chemistry, Vol. 54, No. 6, 1477-1484, 1990) are known as substances with elicitor activity. It is considered that a substance with the elicitor activity may be used as a biotic pesticide. However, it is difficult to put the currently known substances with the elicitor activity into practical use, because there are various problems, such as cost, the amount which can be supplied or the preparation procedure of the polysaccharide as a raw material.
On the other hand, it has been known that a xyloglucan-oligosaccharide which is obtained by degradation of a xyloglucan, i.e., a polysaccharide which constructs the cell wall of a higher plant, participates in growth of a plant. The following facts have been known; that a pentasaccharide, a heptasaccharide or a nonasaccharide of a xyloglucan-oligosaccharide activates endo-1,4-.beta.-glucanase in a plant (Vladimir Farkas et al, Carbohydrate Research, Vol. 184, 213-219, 1988), that a heptasaccharide, an octasaccharide, or a nonasaccharide of a xyloglucan-oligosaccharide promotes elongation of a slice of pea-hypocotyl (Gordon J. McDougall et al, Plant Physiology, Vol. 93, 1042-1048, 1990), and that a nonasaccharide of a xyloglucan-oligosaccharide has an inhibitory effect against elongation caused by auxin (Gordon J. McDougall et al, Plant Physiology, Vol. 89, p. 883-887, 1989). However, the elicitor activity of a xyloglucan-oligosaccharide has not been heretofore known at all.